1. Field of the Invention
The present invention relates to a circuit for detecting voltage of cells in combination for use as a power source for drive motors for electric motor vehicles.
2. Description of the Related Art
Power sources conventionally mounted in electric motor vehicles, such as hybrid cars, for drive motors comprise secondary cells connected in series for use in combination. Because combinations of such cells need to produce a high voltage of 200 to 300 V, for example, 60 to 80 lithium secondary cells each having an output of about 3.6 V are connected in series, or about 200 NiMH secondary cells each having an output of about 1.2 V are connected in series for use in combination.
With the combination of cells, it is desired that all the secondary cells be equivalent in a charged state. Suppose one secondary cell is 70% in charging rate, and another secondary cell is 50% in charging rate. In this case, the amount of electricity chargeable into these cells in combination is 30% which corresponds to the amount of charge for the cell with the charging rate of 70% when it is to be charged to the full. If the two cells are charged to an amount in excess of 30%, the secondary cell with the charging rate of 70% will be charged more than 100% to become greatly shortened in life. Consequently the combination of cells is also shortened in life.
Accordingly, it is practice to monitor the voltages of the individual cells in combination using a voltage monitoring system having the construction shown in FIG. 10. In the case of the system, a plurality of secondary cells are connected in series to provide a cell module, and such modules are connected in series to provide a cell combination 10. Voltage detecting lines extend, respectively, from the opposite terminals of the cell combination 10 and from the points of connection between cell modules, and are connected to a voltage detecting circuit 7. The voltage of the individual cell modules detected by the voltage detecting circuit 7 is fed to an entire control circuit 8. The temperature of the cells is detected by a temperature sensor circuit 81, and the current flowing through the cells is detected by a current sensor circuit 82. The detection results are fed to the entire control circuit 8. The entire control circuit 8 calculates the amounts of electricity remaining in the cell and checks the cells for abnormalities based on the input data The result of monitoring is sent to an external control system through a communication line.
With the voltage monitoring system described, while the cell combination 10 as a whole has a voltage of 200 V to 300 V, a NiMH secondary cell, for example, has a voltage of about 1.2 V. Thus it is difficult to determine the state of individual secondary cells only by monitoring the entire voltage. Consequently, the voltage monitoring system needs to monitor the voltage for at least individual modules.
FIG. 11 shows a conventional voltage detecting circuit 7 for monitoring voltage for each module (JP-A No. 11-160367/1999, 11-160371/1999). With reference to FIG. 11, the cell combination 10 is divided into four cell blocks, each comprising five cell modules. For each cell block, a differential computing circuit 71, an analog-to-digital converter 72, and an insulation interface 73 are connected in series to provide a voltage detecting circuit 7 for detecting a cell voltage for each module. In the case of NiMH secondary cells, one module has a voltage of approximately 12 V.
The differential computing circuit 71 detects the difference of the voltage across each of the cell modules, feeding the detection value to the analog-to-digital converter 72 to have the value converted into a digital value, thereafter feeding the value through the insulation interface 73 having a photocoupler, etc., to the entire control circuit 8. Accordingly, the insulation interface 73 interposed electrically insulates the cell combination 10 of high voltage from the entire control circuit 8.
The differential computing circuit 71, for example, comprises an op-amp, peripheral resistors as shown in FIG. 12 to detect the output voltage of each of the cell modules. For example, an illustrated positive electrode point of a cell module 1 (Module 1) has potential V1 which is a combined amount of voltage of the cell module 1 and the cell module 2 with a negative electrode point of a cell module 2 (Module 2) connected to ground. Opposite terminals of each of the cell modules are connected to the differential computing circuit 71 to detect voltage of each of the cell modules. Then the detected voltage is fed to an analog-to-digital converter 72, obtaining voltage detecting data of each of the cell modules. Accordingly, one of terminals of cell modules is grounded to the earth, to thereby stabilize grounding potential for a cell, realizing accurate differential computing. In this case the analog-to-digital converter 72 shown in FIG. 11 is provided with a grounding.
With reference to FIG. 12, voltage output from each differential computing circuit 71 has a value obtained by multiplying a voltage difference value (V1 to V2) by coefficient A. The coefficient A is so determined at an appropriate value that voltage input to the differential computing circuit 71 falls within the range required for normal AID conversion. For example, in the case where the voltage measured is in the range of 0 to 20 V, and the analog-to-digital converter 72 has an input range of 0 to 5 V, a resistance value of each resistor shown in FIG. 12 is so designed such that A=5 V/20 V=0.25. The resistance values shown in FIG. 12 fulfill the conditions mentioned.
Furthermore, resolution of the analog-to-digital converter 72 is determined according to the resolution required for detecting a voltage of one cell module. When measured voltage of each of the cell modules is as great as up to 20 V, as shown in FIG. 12, for example, and the voltage is detected in resolution of about several dozens of mV, the resolution R is calculated according to Mathematical Expression 1 given below with use of an analog-to-digital converter 72 of 10 bits.R=20 V/210=19.5 mV  (Mathematical Expression 1)
However, the conventional voltage detecting circuit 7 shown in FIGS. 10 to 12 needs to be provided with an op-amp and peripheral resistors for each of the cell modules, thereby giving rise to the problem of making the circuit large in size with an increasing number of the modules.
Furthermore, in the event of the disconnection of the voltage detecting line, the behavior of the circuit shown in FIG. 12 is unknown. Assuming that voltage of a module is 12 V, the correct voltage difference value of 3 V (A=0.25) is fed to the channel CH1 of the analog-to-digital converter 72 when the disconnection does not occur. In the case where the disconnection occurs on a position indicated at a cross mark in FIG. 12, for example, the voltage fed to the channel CH1 of the analog-to-digital converter 72 is 5.3 V, and that to CH2 is about 0.7 V. In this case, it is impossible to distinguish between overcharge of the cell module 1 causing the great voltage differential value input and disconnection causing the abnormal value input. Using the cells in combination continuously in the event of the disconnection may cause damage to the cells, so that this case is to be handled as an abnormal situation. Similarly, in checking voltage of the cell module 2, it is difficult to distinguish between over-discharge causing the small voltage differential value input and disconnection causing the abnormal value input.
Furthermore, with the conventional voltage detecting circuit, the detection accuracy is greatly influenced by the errors of the resistors, thereby presenting difficulty in improving the detection accuracy. That is, highly accurate resistors need to be mounted for improving computing accuracy of the differential computing circuit 71 shown in FIG. 12. Because the accuracy of the resistor is usually about 0.5 to 1%, even if a resistor of 0.5% is adopted, the computing error of the differential computing circuit 71 depends on the error of the combined resistance, and reaches about 1%. Accordingly, in the case of an input value of about 12 V, an error of about 120 mV occurs, whereby such accuracy is unsatisfactory in monitoring voltage of cells each having 1.2 V.
On the other hand, already known for improving the detection accuracy is a method of correction-computing by software. In the case where this method is adopted into the differential computing circuit 71 shown in FIG. 12, the correction difference value (A×V1−B×V2) is calculated in place of the calculation of the voltage difference value (V1−V2). Coefficients A, B each expresses a resistance value error. The smaller the resistance value error, the closer to one the coefficient. However, the voltage difference value for the cell module I is influenced by voltage variations of the cell module 2, for example, necessitating extremely complicated processing for correcting a voltage difference value for each of the cell modules by the software.
An object of the present invention is to provide a voltage detecting circuit which is reduced in the number of parts and which nevertheless improves the voltage detection accuracy with ease, and which further detects reliably disconnection occurrence.